We depend on songbirds to keep the Earth's plant life flourishing. Like the bees, it is the birds who pollinate the flowers and disperse the seeds. They keep insects under control, protecting leaves and seeds and human crops.

But according to the world's leading bird scientists, songbirds are disappearing.

"By some estimates, we may have lost almost half the songbirds that filled the skies almost forty years ago" says respected ornithologist Bridget Stutchbury. Scientists around the globe are in a race against time to discover why this is happening, and what it means.

One species they're studying is the purple martin, whose numbers have dropped by an alarming 78% since 1970, according to the North American Breeding Bird Survey which has been keeping population records for more than fifty years.

The purple martin is a migratory bird that follows the food supply from temperate North America south down to the Amazonian Basin. To learn more about their dramatic downhill slide, Stutchbury and her team band the birds with tiny light-level logging geolocators.

The technology translates sunrise and sunset times into longitude and latitude so Stutchbury knows exactly where the bird has been. But there's a catch; the devices don't send data, they store it, so she needs get the geolocators back once the birds have returned. Miraculously, a few of the martins return with geolocators intact ten months later.

The results she's collected so far are suprising, "we've seen birds that have travelled from Pennsylvania to the Gulf Coast in only two days - 1300 kilometres." The birds are flying much faster than experts thought and her research is shaking up ornithologists' models for songbird migration patterns.

The purple martins leave their wintering grounds at the same time year after year. But they have no idea that thousands of kilometres away, a warming planet has brought on an earlier spring and they may be too late to take advantage of the peak food supply. Understanding the timing of their migration route is critical - with climate change altering the timing of the seasons, the survival of the species is at risk. "Climate change is a new threat for songbirds," says Stutchbury.

To learn about the threats facing songbirds - and the researchers who are studying them - watch SongbirdSOS, a feature film coming to theatres in fall 2014 and to The Nature of Things in spring 2015.